mhansen - 8:30 pm on Dec 17, 2010 (gmt 0)
Could you possibly be in a market that has more search traffic towards the weekend, like football, or some other niche market that would see an increase in different times during the week.
I have a football site I am a part of... Tues-Weds, dead. Thurs, trickle. Friday... here they come! Sat-Monday, crazy-busy! rinse and repeat for football season, over several years.
Check Google insights for the trends in your primary keywords. I know I have been surprised by it before.